將GPS經緯度座標轉換成直線距離

我在網路上找到別人寫好，疑似是C#的版本：

http://www.dotblogs.com.tw/jeff-yeh/archive/2009/02/04/7034.aspx

我改成C/C++的版本：

#define M_PI 3.14159265358979323846
double ConvertDegreeToRadians(double degrees)
{
    return (M_PI/180)*degrees;
}
double GetDistance(double Lat1, double Long1, double Lat2, double Long2)
{
    double Lat1r = ConvertDegreeToRadians(Lat1);
    double Lat2r = ConvertDegreeToRadians(Lat2);
    double Long1r = ConvertDegreeToRadians(Long1);
    double Long2r = ConvertDegreeToRadians(Long2);

    double R = 6371; // Earth's radius (km)
    double d = acos(sin(Lat1r) *
    sin(Lat2r) + cos(Lat1r) *
    cos(Lat2r) *
    cos(Long2r-Long1r)) * R;
    return d;
}

想要使用這個function很簡單
例如想要得知24.797254,120.969189到24.803545,120.979038的距離，就這樣呼叫：

cout<<GetDistance(24.797254,120.969189,24.803545,120.979038)<<endl;

function回傳的單位是公里。